The activation of cells by growth factors, mitogens or other cytokines to undergo proliferation and/or differentiation is often dependent on inducible tyrosine kinase activity. This tyrosine kinase activity increases the phosphotyrosine content of many receptor-like and cytoplasmic regulatory proteins. Often, the physical association of such regulatory proteins is mediated through those phosphotyrosine residues.
For example, one mechanism of cellular regulation involves the physical association of signal transduction proteins with one or more phosphorylated tyrosine-containing receptor subunits, called immunoreceptor tyrosine-based activation motifs (ITAMs), present on its native ligand or receptor. This association is a common feature of many cytoplasmic signal transduction pathways, as well as other immunologically important regulatory protein-receptor based interactions (M. A. Osborne et al., BioTechnology, 13 (1995)). Of particular importance is the interaction between phosphorylated ITAMs and regulatory proteins containing Src homology domain 2 regions ("SH.sub.2 binding domains"). Examples of immunologically important proteins containing SH.sub.2 binding domains include ZAP-70, Fyn, Lyn, Lnk, Abl, Vav, Huk, Blk, PLC.gamma..gamma.l, GAP, Crk, Shc and p.sup.56 lck. Although proteins having SH.sub.2 binding domains tend to have sequence-specific affinities for their ITAM containing ligands or receptors, the binding interaction itself is ubiquitously mediated through one or more phosphorylated tyrosine residues. Therefore, the presence of phosphorylated tyrosine plays a critical role in signal transduction involving virtually all proteins containing SH.sub.2 binding domains.
Given the above understanding of signal transduction and cellular regulation, it follows that growth factor- and cytokine-induced cell proliferation and/or differentiation can be selectively inhibited by antagonizing the interaction of regulatory proteins dependent on tyrosine kinase activity with their native phosphotyrosine-containing ligands or receptors. Such antagonists would undoubtedly be useful to treat a variety of disorders, including those associated with or caused by neoplastic diseases or chronic inflammatory diseases.
To date, however, antagonists of tyrosine kinase-dependent regulatory proteins have not fulfilled their potential as useful pharmaceutical agents. One major hurdle has been the necessary inclusion of a phosphorylated .alpha.-amino acid residue or a phosphorylated analog thereof, to perform the crucial role of the native tyrosine phosphoprotein ligands or receptors of these regulatory proteins. However, agents containing phosphotyrosine, other phosphorylated .alpha.-amino acid residues, or phosphorylated analogs thereof, cannot generally be used as therapeutic agents because the presence of the phosphorylated moiety substantially impedes cell penetrability. Until now, no effective replacement or mimic for the critical phosphotyrosine residue has been identified. Further, no accurate methods for identifying such phosphotyrosine mimics have existed. Accordingly, the need exists for effective phosphotyrosine mimics and convenient methods for identifying such mimics.